Wire thermoelements, particularly miniaturized wire thermoelements are known. Two wires, for example a platinum wire and a platinum-rhodium alloy wire are introduced into a capillary opening of a ceramic body, which ceramic body-wire combination is then introduced into a metal tube. The structure is then worked, for example being subjected to drawing and/or forging steps.
The structures are tiny. For example, the wires may have a diameter of about 0.1 millimeter, and the structure, when finished, has an outer diameter of less than 1 mm and an axial length of, for example, about 5 cm.
Jacketed thermoelements usually are temperature sensors in which two wires, suitable to form a thermocouple are connected together at one end, for example by welding, and introduced into a ceramic body which separates the wires from each other throughout the length of the thermoelement and, additionally, separates the wires from a metal jacket. The ceramic structure, thus, provides for insulation of the wires--except at the welded thermal junction, as well as from the metal jacket.
To manufacture jacketed thermoelements, it is customary to start with a tubular element of ceramic, for example magnesium oxide. The ceramic tube has two capillary openings therein. The wires which are to form the thermocouple are threaded into the capillaries. Typical thermocouple combinations are platinum and a platinum-rhodium alloy; or nickel and a nickel-chromium alloy. For the Pt-PtRh combination, platinum of high purity, for example of about 99.99% is desirable. The ceramic tube is fitted into a tubular metal jacket, for example of a platinum-rhodium alloy, e.g., PtRh 10; another suitable material is Inconel (Reg. TM of the INCO COMPANY). The so constructed sub-assembly is then deformed, by mechanical working to the desired dimension. Typical working is by hammering, carried out uniformly from all the sides so that the geometry of the structure is retained, or by drawing.
The metallic structures of the sub-assembly can be plastically deformed; in contrast thereto, however, the ceramic tube cannot be deformed. The ceramic breaks upon the mechanical working and pulverizes to become a very fine grained powder, which tightly surrounds and embeds the wire elements. It is customary to use fine grain ceramic, with a theoretically possible density up to about 95%.
The effect of mechanical working is transferred from the jacket via the ceramic to the thermocouple wire elements. Stresses introduced into the jacket by the mechanical working and deformation thereof, as well as frictional strains due to engagement with a drawing die, are not directly applied to the thermocouple wire elements; rather, the thermocouple wires are stressed indirectly by transfer of stresses by the ceramic particles. The ceramic particles, themselves, cannot be plastically deformed or be subjected to metal flowing. The wires are not centrally located within the tubular structure; thus, tensions and stresses extending from the jacket up to the center of the core are different at different radial positions. This causes the wires to lose their geometry and to break. Breakage is particularly severe if thermoelements of extremely small size are to be made, for example thermoelements having a diameter which is less than 0.4 mm outer diameter, and especially less than 0.25 mm outer diameter.
Typical jacketed thermoelements are described in the referenced publication: VDI-Berichte Nr. 112, 1966, Seite 95 ff. (DK 536.532-213.3) unter der Uberschrift "Mantel-Thermoelemente" von Ing. T. Sannes, Eindhoven (Niederl.), (Reports by the Society of German Engineers, No. 112 (1966) page 95 et. seq., article by Ing. T. Sannes, Eindhoven (Netherlands) entitled: "Jacketed Thermoelements"). This publication describes that thermoelements of only 0.34 mm outer diameter have been made to be used in nuclear technology, having a flat-rolled measuring end point. Such thermoelements did not use a platinum-platinum/rhodium 10 thermocouple, since thermal wires of the platinum-platinum/rhodium 10 combination are not suitable for use in nuclear reactor technology.
Other jacketed thermoelements are described in "elektrotechnik" 52, H. 4, 18, 02.70, Seiten 16 und 17 unter der e,uml/u/ berschift "Thermoelemente miniaturisisiert und gut bewehrt" (Electrotechnik, Issue 52, 18. Feb. 1970, pages 16, 17, article "Miniaturized Thermoelements, Well Proven"). This publication stresses that for small diameter, a trend to use thermal wires of chrome-alumel has been noticed, since this combination of materials has more desirable characteristics and can provide thermocouples of minimum dimension--outer dimensions to down to 0.25 mm. Diameters and dimensions for platinum, and platinum-rhodium thermocouples, as well as methods for the manufacture of the thermocouples are not disclosed; the platinum-platinum/rhodium combination is mentioned, but is not stated how they can be made.